Minimum stressed wrench

ABSTRACT

A wrench for use on polygonal fasteners, which wrench grips the fastener in such a way that maximum leverage (moment-arm) is obtained for turning the fastener, even at the sacrifice of marring the fastener in the process. The fastener gripping surfaces being roughened.

United States Patent 1191 Andersen Sept. 9, 1975 [54] MINIMUM STRESSED WRENCH 2,959,995 11/1960 Linden 81/119 ux 3,242,775 3 I966 H' kl SI 12! R X [76] Inventor: Alfred Frederick Andersen, 2OI4 m e I Los Angeles Ave., Berkeley, Calif. FOREIGN PATENTS OR APPLICATIONS 94707 3,886 8/1882 United Kingdom 81/119 [22] Filed: Nov. 14, 1973 Primary Examiner-AI Lawrence Smith [2|] App]. No.: 415,781 Assistant Examiner]ames G. Smith [52] US. Cl. 81/121 R; 81/186 57 ABSTRACT [51] Int. Cl. B25B 13/06 A h f l l f t h [58] Field 61 Search 81/120, 121 R, 121 B, 186, use F mm as Y" 8H1 19 wrench grlps the fastener in such a way that maxlmum leverage (moment-arm) is obtained for turning the [56] References Cited fastener, even at the sacrifice otlmarrmg the fastener 1n the process. The fastener gripping surfaces being UNITED STATES PATENTS r0ughened 1.590200 6/1926 McGuckin 81/121 R x 2,777,348 1/1957 Wraight 81/186 6 Claims, 6 Drawmg Flgures PATENTED 9W5 3,903,764

SHEET 1 x \TORQUE T MA (7 AN\3O") .577 F; TYPICAL FORCE EXEETED BY WRBVCH ON FASTENER Ff TYPICAL. FORCE EXQQTED BY FA8TENER ON WRENCH TORQUE =7" PATENTED SEP 9W5 3.903.764

sum 2 o 5 PATENTEB SEP 9 I975 SHEETHQ5 MINIMUM STRESSED WRENCH SUMMARY OF THE INVENTION In the past, wrenches have generally been designed to engage the fastener in such a way that the fastener is turned without marring the surface of the fastener, if possible. Thus, except in cases where serrations are needed to prevent slipping, such as with pipe wrenches and pliers, where the object being turned is essentially round, the gripping faces of wrenches have understandably been smooth; and in cases where the wrench cavity completely surrounds the fastener, such as in box wrenches and socket wrenches, and thus where there is no danger of Slipping, there has been no thought of putting serrations on the gripping faces. Thus, all box and socket wrenches have been designed and manufactured with generally smooth faces. And this practice has been only sensible and rational considering that the avoidance of slipping and marring, where possible, has been practically the sole concern in wrench designs up to this time.

But the wrench of this invention is designed for quite a different purpose; namely, to so grip the fastener that minimum stress is generated in the wrench, even at the expense of marring the fastener. Despite the fact that it applies to wrenches of the closed type, such as box and socket wrenches, and thus presents no need to employ serrations or other means to avoid slipping, it employs such serrations and other means in order to transmit the applied torque by way of an enlarged momentarm, which in turn results in a greatly reduced reactive force on the wrench and reduced reactive stresses in the wrench.

In the preferred form of this invention the serrations, or other means of generating friction between the faces of the wrench and the fastener, are designed so as to increase the friction, and thus increase the moment-arm, as applied torque is increased, so that maximum moment-arm is attained at the time when applied torque is maximum.

The practical advantage of reducing reactive stresses in the wrench is in then making it possible to manufacture the wrench out of less expensive material with less tensile strength, or to reduce the outside diameter of the wrench, thus making it possible to engage fasteners where clearance around the fastener is insufficient to allow entry of conventional wrenches.

The details of how the wrench of this invention engages the fastener so as to bring about greatly reduced stresses in the wrench is explained in more detail in the following discussion of the drawings.

IN THE DRAWINGS FIG. 1 shows a typical section of a conventional 12- point socket, 18, and a section of a regular hexagon fastener, 19, where said socket is applying a clockwise torque, T, to said fastener, which torque is transmitted by way of 6 forces, of which F, is typical, and which forces impinge on said fastener at 6 equally spaced portions of the fastener, of which portions portion A, is typical, and where both the wrench cavity and the fastener are exactly nominal size and thus fit each other exactly.

FIG. 2 shows the same as FIG. 1 except that the wrench cavity is shown at approximately the maximum manufacturing tolerance and the fastener is shown at approximately the minimum manufacturing tolerance,

the resulting engagement being at the corner of the fastener as shown.

FIG. 3 shows a section of a regular hexagon fastener, 20, and a section of an unconventional wrench cavity with lugs on the gripping faces, of which the lug shown, 25, is typical.

FIG. 3a shows a section of a regular hexagon fastener, 47, a section of a wrench of this invention, 46, and a typical engagement of the fastener by the wrench at 41, 42, 43, 44, and 45 by way of typical forces f,,, f,,, f,,, f,., and f,, respectively at a time when indentation is taking place under the application of substantial torque.

FIG. 4 shows a section, 43, of a variation of the wrench of this invention, where the wrench surfaces employed in turning the fastener clockwise are smooth, whereas the surfaces employed in turning the fastener counterclockwise are roughened in some way so as to generate friction between the impinging surfaces.

FIG. 5 shows a cross-section of a typical wrench of this invention, 50, engaging a typical fastener, 51.

THE DRAWINGS IN DETAIL Again referring to FIG. 1, F is a typical reactive force, equal and opposite to F,, impinging on said socket wrench at a typical portion, A,, of said wrench. The typical wrench and fastener surfaces, extending from 12 to A are smooth, as in all conventional wrenches of the closed type, and therefore can offer no appreciable resistance to sliding on each other. Thus, there can be no appreciable force-component in the direction from 12 to A, or from A, to 12. Thus, F, and F must, as shown, be essentially perpendicular to the direction determined by the impinging surfaces. That is, F, and F must, as shown, be along the line determined by A, and 17, and thus the perpendicular distance from 0,, the geometric center of both the fastener and the wrench cavity for the plane shown, to the line determined by A, and 17 must be, as shown, the distance from O, to 17, and this distance, then, is the magnitude of each of the moment-arms by means of which the 6 said forces transmit said applied torque, T, to said fastener, 19.

The magnitude of said moment-arm is, applying trigonometry, equal, as shown, to NS/2(Tan 30) as a maximum attainable without friction between the impinging surfaces, where NS is abbreviation for Nominal Size, and is the nominal perpendicular distance across the flats of the fastener.

Thus, if the total torque applied is T, then T 6(F, NS/2 Tan 30) Removing the fraction we get:

T 3F,(NS)Tan 30 Dividing both sides by 3(NS)Tan 30:

F, T/3(NS)Tan 30 Rephrasing we get:

F, T Cot 30/3(NS) Substituting 1.733 for Cot 30:

F, (1.733)T/3(NS) Dividing we get:

Such, then, is the magnitude of a reactive force repre sentative of the six reactive forces impinging on a conventional socket-type wrench of perfect fit with an hexagonal fastener.

Such a perfect fit is, of course, impractical in practice because no provision is made for clearance for engagement and disengagement. The dotted lines in FIG. 1 indicate the lines along which the wrench surfaces would more likely lie in practice, in order to provide clearance for engagement and disengagement. Thus, in practice, the wedging action tending to spread the typical conventional socket-type wrench is typically greater than that shown in FIG. 1, and might even be as great as shown in FIG. 2.

Referring to FIG. 2, if we take the moment-arrn here to be roughly, 48(NS/2), as shown, and perfonn the same trigonometric calculations to determine F as were performed to determine F we will find that P}; will equal F (0.577/0.48), or approximately 0.7T/NS. Therefore, it is unlikely that the typical reactive force impinging on a typical conventional socket-type wrench will be under the magnitude of 0.6T/NS. We keep this in mind as we proceed to calculate what the typical comparable reactive forces would be in a wrench of this invention.

Referring to FIG. 3, we see here a way to secure the maximum possible moment-arm by having the wrench exert tangential force only, with no radial component. The result is a moment-arm more than twice the magnitude obtained, as shown in FIGS. 1 and 2, in the typical conventional socket-type wrench. Accordingly, the reactive force F would be less than half of even the impractical conventional ideal, F But the wrench sug gested in FIG. 3 would be impractical from another standpoint; namely, that its lugs would tend to sheer off the corners of the fastener, such as at 22, and would se verely mar the fastener. This brings us close to the compromise solution which constitutes the essential feature of the wrench of this invention.

Referring now to FIG. 30, we see here a modification of FIG. 3. All of the forces impinging on the fastener, and all the reactive forces impinging on the wrench are, as in FIG. 3, tangential forces, utilizing the maximum practical moment-arm, but instead of only one lug, there are several, the number varying with circumstances and with desirable outcomes. The estimated resultant of all these forces, F therefore calculates to be roughly half of what can be expected in the typical conventional socket-type wrench. Note that the surfaces of the lugs by means of which these forces impinge on the fastener all lie along planes which pass through the common center, of the wrench and the fastener. It is this feature which assures that none of the forces will have radial components, the significance of which will be discussed subsequently. Thus, the gripping faces of said lugs are not parallel to each other, and are not serrations in the common usage of that word. Strictly speaking, they are a series of lugs, each angled with respect to the common center, rather than to each other.

These lugs can be sized and spaced such that when only a minor torque is applied to the fastener the lugs, shown at 41, 42, 43, 44, and 45 as typical, would not penetrate the fastener, in which case the moment-arms and resultant forces would be roughly as shown and calculated in the discussion of FIG. I. However, when substantial torque was applied, then the lugs would penetrate; the greater the torque, the greater the penetration. Thus, these lugs could be sized and spaced so that even though torque was substantially increased, such increase would not increase the reactive forces reacting on the wrench, and might even serve to decrease them.

Since F is roughly half of F the reactive stresses set up in the wrench by F would, all other things being equal, be roughly half those set up by F But all other things are not equal, for there is a further advantage of F over F namely, the direction in which it impinges on the fastener, which determines in turn the direction of the reactive forces on the wrench, and thus the direction of the estimated resultant reactive force, F The explanation is as follows:

The direction of F being largely radial, sets up bending moments, and the stresses accompanying these, in the thin sections of the wrench wall, whereas F having no radial component whatever, avoids these bending moments completely, for it is completely tangential. Thus, whereas the forces set up in a conventional socket-type wrench tend to change the shape of the wrench, and thereby generates substantial bendingmoment stresses, the forces set up in the wrench of this invention do not tend to change the shape of the wrench: that is, not at the time of maximum applied torque, which is the time when fracture would occur if it is to occur.

Therefore, the stresses in the wrench of this invention would, from a complex of factors, be only /a, or perhaps only A, as great as the stresses set up in the conventional wrench of the socket-type, when maximum torque is applied.

The wrench illustrated in FIG. 4 is a variation on the wrench of this invention, where in retaining smoothness for those surfaces employed in turning the fastener clockwise, generally the tightening direction, and the direction in which maximum torque is not generally applied, moment-arm is sacrificed in order to completely remove the possibility of marring the fastener. However, sorne means of generating friction between the impinging surfaces is retained for those wrench surfaces employed in loosening, for it is in loosening, such as in loosening a corroded fastener, that the maximum torque is generally applied, and thus it is in the loosening (counterclockwise) direction that the frictional means contributed by the wrench of this invention are most called for.

FIG. 5 shows the general form of the wrench of this invention, 50, engaging a hexagon fastener, S], to turn said fastener clockwise by way of gripping surfaces of which 52 is typical, and where some frictional means are incorporated into said gripping surfaces, the nature of which frictional means may vary from one practical application to another and may vary between those surfaces employed in turning the fastener clockwise, of which surface 52 is typical, and those surfaces employed in turning the fastener counterclockwise, of which surface 53 is typical, for the reasons stated above.

The frictional means, other than a series of lugs, referred to above, might include impregnating the wrench surfaces with sand particles, carbide particles, or some other hard particles, or simply roughing up said surfaces sufficient to generate substantial friction.

I claim:

1. A wrench for turning a nut or bolt of regular polyg onal shape. said wrench having a closed cavity of subsurfaces for engaging and turning said nut or bolt in either a clockwise or counterclockwise direction of rotation about the central axis of said regular polygonal shape, which subsurfaces are symmetrical with respect to said axis when engaging said nut in either direction of rotation, and at which times the engaging subsurfaces of said cavity are all essentially parallel to said axis, and where during turning there are one or more sets of such subsurfaces employed exclusively in turning said nut in the clockwise direction and one or more sets employed exclusively in turning said nut in the counterclockwise direction of rotation, each such set having at least one pair of opposing subsurfaces, and where one or more of said subsurfaces are roughened to substantially increase the frictional resistance to slipping between such subsurfaces and the faces of a nut or bolt gripped by said subsurfaces.

2. A wrench of claim I, where the roughened gripping surfaces incorporate a series of sharp lugs.

3. A wrench of claim 1, where the roughened gripping surfaces incorporate a series of sharp lugs whose gripping faces lie in planes which pass through the geometric center of the wrench cavity.

4. A wrench of claim 1, where the roughened gripping surfaces incorporate a series of sharp lugs spaced so close together that no appreciable indentation in the fastener takes place except under very substantial torque approaching maximum torque.

5. A wrench of claim 1, where the roughened gripping surfaces incorporate impregnated hard particles.

6. A wrench of claim 1, where the roughened gripping surfaces employed in turning a fastener clockwise are substantially less rough than those employed in turning it counterclockwise. 

1. A wrench for turning a nut or bolt of regular polygonal shape, said wrench having a closed cavity of subsurfaces for engaging and turning said nut or bolt in either a clockwise or counterclockwise direction of rotation about the central axis of said regular polygonal shape, which subsurfaces are symmetrical with respect to said axis when engaging said nut in either direction of rotation, and at which times the engaging subsurfaces of said cavity are all essentially parallel to said axis, and where during turning there are one or more sets of such subsurfaces employed exclusively in turning said nut in the clockwise direction and one or more sets employed exclusively in turning said nut in the counterclockwise direction of rotation, each such set having at least one pair of opposing subsurfaces, and where one or more of said subsurfaces are roughened to substantially increase the frictional resistance to slipping between such subsurfaces and the faces of a nut or bolt gripped by said subsurfaces.
 2. A wrench of claim 1, where the roughened gripping surfaces incorporate a series of sharp lugs.
 3. A wrench of claim 1, where the roughened gripping surfaces incorporate a series of sharp lugs whose gripping faces lie in planes which pass through the geometric center of the wrench cavity.
 4. A wrench of claim 1, where the roughened gripping surfaces incorporate a series of sharp lugs spaced so close together that no appreciable indentation in the fastener takes place except under very substantial torque approaching maximum torque.
 5. A wrench of claim 1, where the roughened gripping surfaces incorporate impregnated hard particles.
 6. A wrench of claim 1, where the roughened gripping surfaces employed in turning a fastener clockwise are substantially less rough than those employed in turning it counterclockwise. 